Marijuana (cannabis) use by pregnant women in the US is increasing. Yet, we do not know if this use is safe for the fetus. A recent comprehensive literature review by the National Academy of Sciences, Engineering and Medicine found substantial evidence of an association between marijuana use during pregnancy and lower birth weight, but only limited evidence of other health problems related to prenatal, perinatal or neonatal outcomes of marijuana use during pregnancy. However, they noted a number of limitations of the reviewed studies such as reliance on self-reporting to ascertain marijuana exposure, the varying potency, dosage and timing of marijuana exposure, limited statistical power to detect many outcomes, and marijuana exposure confounded by the use of other substances namely tobacco and alcohol. To understand fetal safety of marijuana use during pregnancy, animal studies were conducted and showed that fetal exposure to ?9-tetrahydrocannabinol (THC), the most abundant and psychoactive cannabinoid, caused lower birth weight, increased fetal resorption and even in utero deaths in animals. THC also inhibits human placental trophoblast proliferation and differentiation. However, most of the animal and in vitro studies were conducted with high THC doses or concentrations that cannot be used or observed in humans and therefore the data obtained cannot be used to predict human toxicity. Also, fetal plasma/tissue THC concentrations, after maternal marijuana use, are unknown. Given the limitations of human observational, animal and in vitro studies, alternative approaches to determine fetal exposure and risks of marijuana use during pregnancy need to be explored. The overall goals of this P01 are to predict maternal-fetal exposure to THC and its psychoactive metabolite 11-OH-THC through innovative in vitro and in vivo studies integrated through maternal-fetal-Physiologically Based Pharmacokinetic (m-f-PBPK) modeling and simulations (M & S) and conduct exploratory studies that will inform fetal neurotoxicity of THC/11- OH-THC that could result in long-term sequelae. To achieve these goals, it is imperative that we first understand factors that determine fetal exposure to THC/11-OH-THC, the focus of this project (Project 1). Based on literature data and our preliminary studies, we hypothesize that placental P-gp, BCRP and CYP1A1 work in tandem to modulate fetal exposure to THC/11-OH-THC. To test this hypothesis, Project 1 will characterize THC/11-OH- THC metabolism and transport in relevant maternal organs (intestine and liver), placenta and fetal tissues. To verify if efflux transporters in the placenta are involved in trans-placental transfer of THC/11-OH-THC, we will also perform ex vivo human placental perfusion and in vivo maternal/fetal pharmacokinetic studies in mouse models. These studies address a critical gap in our understanding of THC/11-OH-THC metabolism and transport clearances in humans and will provide data to Projects 2 & 3 to allow prediction of maternal-fetal exposure to THC/11-OH-THC through PBPK M & S. Data generated by this project will also inform future relevant animal and in vitro toxicity studies to assess fetal and long-term developmental toxicity of THC/11-OH-THC.